Epidemiologic and experimental data have established the adverse neurobehavioral effects of exposure in utero and during early childhood to numerous environmental toxicants, including lead, alcohol, mercury, PCB s, and environmental tobacco smoke (ETS). Still, the ideal biomarker for measuring in utero exposure to specific toxicants has not been established and the adverse effects of many potential neurotoxicants have not been rigorously tested. Fetal exposure is typically measured with self-reported surveys, maternal blood and urine, or cord blood. In contrast, meconium is a non-invasive method to simultaneously test for chronic exposures to numerous toxicants, but it is as yet unclear whether conventional biomarkers or meconium are more predictive of the adverse effects linked with specific toxicants. For lead exposure, emerging data indicate that our efforts should emphasize primary prevention, but the safety and efficacy of lead hazard controls are uncertain, especially for children with low blood lead concentrations. The investigators propose to conduct a longitudinal cohort study of 400 children, followed from less than 16 weeks gestation to 36 months of age, to examine the dose-response of low-level exposures (pre- and postnatal) to prevalent neurotoxicants with neurobehavioral outcomes and development disorders, such as conduct disorder, cognitive deficits, hearing loss and behaviors consistent with ADHD. The investigators will also conduct a nested, randomized controlled trial to test the efficacy of lead hazard controls or the development of adverse neurobehavioral effects. In Project 1 of University of California's Center Grant, the investigators will test the following hypotheses: 1) In utero exposures as measured by survey (alcohol and ETS), maternal and cord blood (lead, mercury, pesticides and ETS), and urine (pesticides) are less predictive of in utero effects of prevalent toxicants, such as cognition, behavioral problems, growth and hearing, compared with the same toxicants in meconium. 2) Postnatal exposures to pesticides, ETS and lead (at blood levels below 10 ug/dl) are associated with adverse neurobehavioral effects, growth delay and hearing loss in early childhood. 3) Children in the Lead Reduction Group will have blood lead levels that are 2.7 ug/dl (30%) or lower, significantly higher cognitive scores, less hearing loss, greater growth velocity, and fewer behavioral problems than the Control Group at 36 months of age. If funded, this project, combined with the specific aim of Project 2, to validate meconium as a marker of in utero exposure, will serve as a model to evaluate the adverse effects of exposures to multiple prevalent toxicants, validate meconium as a measure of in utero exposures to numerous toxicants, provide exposure and risk assessment data for residential pesticides, test an intervention for the primary prevention of subclinical lead toxicity, and test the efficacy of a lead hazard control on children?s blood lead concentration and neurobehavioral functioning.